1. Field of the Invention
The present invention relates to a polishing liquid supply apparatus for use in a polishing unit for polishing a surface of a workpiece such as a semiconductor wafer, and more particularly to a polishing liquid supply apparatus which is capable of supplying a polishing liquid that stably contains an additive.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 xcexcm wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus.
FIG. 5 of the accompanying drawings shows a conventional polishing unit. As shown in FIG. 5, the conventional polishing unit comprises a turntable 142 with a polishing cloth 140 attached to an upper surface thereof, a top ring 144 for holding a semiconductor wafer W which is a workpiece to be polished while rotating the semiconductor wafer W and pressing the semiconductor wafer W against the polishing cloth 140, and a polishing liquid supply nozzle 146 for supplying a polishing liquid Q to the polishing cloth 140. The top ring 144 is connected to a top ring drive shaft 148, and vertically movably supported by an air cylinder (not shown).
The top ring 144 supports on its lower surface an elastic pad 150 made of polyurethane or the like. The semiconductor wafer W is held on the top ring 144 in intimate contact with the elastic pad 150. The top ring 144 also has a cylindrical guide ring 152 mounted on a lower outer circumferential surface thereof for preventing the semiconductor wafer W from being dislodged from the lower surface of the top ring 144 while the semiconductor wafer W is being polished. The guide ring 152 is fixed to the top ring 144, and has a lower end projecting downwardly beyond the lower holding surface of the top ring 144 to define a recess between the lower holding surface of the top ring 144 and the projecting lower end of the guide ring 152 for holding the semiconductor wafer W therein.
With the above structure, the semiconductor wafer W is held against the lower surface of the elastic pad 150 on the lower surface of the top ring 144, and pressed against the polishing cloth 140 by the top ring 144. The turntable 142 and the top ring 144 are rotated about their own axes to move the polishing cloth 140 and the semiconductor wafer W relatively to each other for thereby polishing the semiconductor wafer W. At this time, the polishing liquid Q is supplied from the polishing liquid supply nozzle 146 to the polishing cloth 140. The polishing liquid Q comprises fine abrasive particles suspended in, for example, an alkaline solution. Therefore, the semiconductor wafer W is polished by a composite action of a chemical action of the alkaline solution and a mechanical action of the fine abrasive particles. Such a polishing process is referred to as chemical mechanical polishing (CMP).
In order to polish the semiconductor wafer W satisfactorily by the polishing apparatus, it is necessary that the polishing liquid having a constant concentration be supplied stably at a constant rate to the polishing unit. The polishing liquid is supplied from a polishing liquid supply system which includes a raw material tank for storing a raw material that comprises a mixture of KOH, NH4OH, or the like, and powder silica, and an adjustment tank for adjusting the raw material supplied from the raw material tank to a predetermined concentration by diluting the raw material with pure water or a chemical solution. The polishing liquid supply system further includes a supply tank for temporarily storing a polishing liquid adjusted by the adjustment tank and supplying the polishing liquid, and a polishing liquid supply piping system interconnecting the tanks for supplying the polishing liquid from the supply tank to the polishing liquid supply nozzle 146 in the abrasive unit.
The polishing liquid contains an additive such as an oxidizing agent for modifying or reforming the polished surface of the semiconductor wafer. Specifically, an oxidizing agent such as H2O2 (hydrogen peroxide) is added for the purpose of oxidizing a metal film of copper or tungsten that has been deposited on the semiconductor wafer. It has been customary to add the additive when the polishing liquid is produced. Thus, in the case where the additive, like oxidizing agent, added to the polishing liquid is chemically unstable, the properties of the polishing liquid tend to be changed when the polishing liquid with the additive is held in stock for a long period of time, with the result that the polishing capability of the polishing liquid becomes unstable.
If the additive is added to the polishing liquid in the polishing liquid supply system, then it has been the general practice to supply the additive, which has been diluted to a desired concentration with a solvent such as pure water in a polyethylene container or the like, from an additive supply unit to the polishing liquid supply system. Thus, the additive supply unit is relatively large in size, and needs a large installation space. In addition, because the additive is added in a small quantity to the polishing liquid in the polishing liquid supply system, the additive is required to be supplied highly accurately.
It is therefore an object of the present invention to provide a polishing liquid supply apparatus which is capable of supplying a polishing liquid that stably contains an additive to a polishing unit, and a polishing apparatus having such polishing liquid supply apparatus.
According to an aspect of the present invention, there is provided an apparatus for supplying a polishing liquid to a polishing unit for polishing a workpiece, comprising: a supply tank for storing a polishing liquid having a predetermined concentration; a polishing liquid pipe for delivering the polishing liquid from the supply tank to a polishing liquid supply nozzle in the polishing unit; an additive tank for storing an additive having a predetermined concentration; and an additive supply pipe for adding the additive supplied from the additive tank to the polishing liquid stored in the supply tank or to the polishing liquid in a polishing liquid passage including the polishing liquid pipe.
Since the additive is added to the polishing liquid at a position close to the polishing unit where the polishing liquid is used, the polishing liquid that contains the additive of stable quality at a required concentration can be supplied to the polishing unit even if the additive comprises an oxidizing agent or the like that tends to be easily degraded or decomposed due to aging. Therefore, the polishing unit can polish a workpiece stably in a high quality with the polishing liquid. The additive may comprise an oxidizing agent such as iron nitrate, an aqueous solution of hydrogen peroxide or ammonium persulfate, or material of stabilizing the distribution of particle diameters of abrasive particles in the polishing liquid.
The polishing liquid supply apparatus may further comprise an additive concentration adjusting device for adjusting the concentration of the additive. The additive concentration adjusting device may comprise an additive preparing device for mixing a raw material powder and a solvent to form the additive. Inasmuch as the additive is prepared from its raw material immediately before it is used, the additive is prevented from being degraded or decomposed due to aging.
The polishing liquid supply apparatus may further comprise an additive concentration adjusting device associated with the additive tank or the additive supply pipe, for adjusting the concentration of the additive, or an additive quantity adjusting device associated with the additive tank or the additive supply pipe, for adjusting the quantity of the additive which is added to the polishing liquid. The additive concentration adjusting device and/or the additive quantity adjusting device are effective to keep the concentration of the additive in the polishing liquid at a desired value so as to meet the required conditions.
The polishing liquid supply apparatus may further comprise a sensor associated with the polishing liquid pipe for detecting the concentration of the additive, and a controller for controlling the additive concentration adjusting device in response to an output signal from the sensor. Further, the polishing liquid supply apparatus may further comprise a sensor associated with the polishing liquid pipe for detecting the concentration of the additive, and a controller for controlling the additive quantity adjusting device in response to an output signal from the sensor.
According to another aspect of the present invention, there is provided a polishing apparatus for polishing a workpiece, comprising: a polishing unit for polishing a workpiece by holding the workpiece by a workpiece holder and pressing the workpiece against a polishing surface on a polishing table; and a polishing liquid supply apparatus for supplying a polishing liquid to the polishing unit, the polishing liquid supply apparatus comprising: a supply tank for storing a polishing liquid having a predetermined concentration; a polishing liquid pipe for delivering the polishing liquid from the supply tank to a polishing liquid supply nozzle in the polishing unit; an additive tank for storing an additive having a predetermined concentration; and an additive supply pipe for adding the additive supplied from the additive tank to the polishing liquid stored in the supply tank or to the polishing liquid in a polishing liquid passage including the polishing liquid pipe.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.